Automated product assembly and manufacturing processes often rely on machine vision to determine the position of a component being processed. Typically, a linear pattern, such as a cross-hair, is used as a reference point for precision alignment.
Various methods of machine vision positioning are currently practiced. Among the primary ones are variations on the general Hough transform and correlation (template matching). With respect to the latter, there exists binary exclusive-OR correlation and, more recently, gray-scale normalized correlation. One common element of these prior art methods is that they require, as input, an image template of the pattern they are to locate.
As a component undergoes various stages of assembly or manufacture, the appearance of the alignment pattern may change. For example, etching and masking of a semiconductor wafer can alter the contrast, definition and thickness of cross-hairs embedded in the wafer. Thus, in order to be effective during the entire assembly process, the prior art methods typically require many image templates. Unfortunately, the actual appearance of a locating pattern on a component is not always predictable, frustrating even the use of multiple ideal template images.
A further drawback of prior art systems is that they consume excessive time in comparing actual optical images to the multiple template images.
In view of the foregoing, an object of the invention is to provide an improved vision system and, more particularly, improved methods and apparatus for accurately locating the center of a linear pattern, e.g., a line or a cross-hair, in an optical image.
Still another object of the invention is to provide a machine vision positioning system that does not rely on an object template in order to position a piece.
Yet another object of the invention is to provide a system capable of positioning a component notwithstanding changes in its appearance during processing.